This invention relates to medical diagnostic equipment and methods and is particularly concerned with measurement of carbon dioxide in the body. The invention more particularly relates to an electrochemical pCO.sub.2 sensor for making in vivo measurements.
A fall in the intramucosal pH may precede the development of intestinal ischemia and stress ulceration. The fall in pH in intestinal mucosa can be reliably calculated from a pCO.sub.2 (partial pressure of CO.sub.2) measurement using the Henderson-Hasselbalch equation. This is described in commonly assigned U.S. Pat. No. 4,643,192, entitled "Hollow Viscous Tonometry" and continuation applications Ser. Nos. 120,720 and 233,888 and in commonly assigned patent applications Ser. Nos. 237,287 and 380,706 entitled "Remote Sensing Tonometric Catheter Apparatus and Method," and in Ser. No. 237,286 entitled "Tonometric Catheter Combination," which are herein incorporated by reference.
In the above body of commonly assigned patent and applications certain techniques for sensing pCO.sub.2 in vivo are described. Although semiconductor CHEMFET sensors and optical sensors are used, it would also be desirable to use electrochemical sensors for pCO.sub.2 measurement. We have attempted to use conventional, commercially available electrochemical pCO.sub.2 sensors and have found them to be generally inadequate. Conventional electrochemical pCO.sub.2 sensors, based on the Severinghaus technology, are not sufficiently miniaturized or constructed for use in vivo do not give accurate readings unless properly oriented. For example, many electrochemical pCO.sub.2 sensors work properly only when physically oriented upright. They do not work properly when oriented on the side or upside down.
Addressing the needs of the biomedical community, the present invention overcomes the shortcomings of the prior art electrochemical pCO.sub.2 sensor technology. The present invention provides a miniaturized pCO.sub.2 sensor of the electrochemical type. The sensor is well adapted for actual insertion through an appropriate orifice into a hollow organ for in vivo pCO.sub.2 measurement. The probe comprises a selectively permeable first membrane which defines a closed reference chamber. A first electrode is disposed in the reference chamber. The first membrane is preferably a bulb-like glass membrane which is permeable to H.sup.+ ions. A hollow flexible tube has a fluid tight plug inserted fully into one end leaving a test chamber space between the plug and the open end. An end cap comprising a selectively permeable second membrane is fitted over the open end to define a closed test chamber between end cap and plug. Preferably the second membrane comprises a silicone rubber which is permeable to CO.sub.2 molecules.
The first membrane, with first electrode enclosed, is disposed within the test chamber. A second electrode is disposed in the test chamber in proximity to the first membrane. Preferably the second electrode is wrapped about the bulb-like first membrane structure adjacent the first electrode. The reference chamber is filled with a first electrolyte of known H.sup.+ ion concentration. HCl may be used for this purpose. The test chamber is filled with a second electrolyte preferably including bicarbonate ions. The test chamber and reference chamber are both of such a size, and the respective electrolytes are of such a volume, that the respective electrolytes remains in contact with the corresponding electrodes over all spatial orientations of the probe. In other words, even when inserted through a body orifice, as in the case of a naso-gastric measurement, both electrodes remain in proper contact with their respective electrolytes, regardless of probe orientation. Accurate results are achieved whether the probe is right side up, upside down, or somewhere between.
The probe construction of the invention is well adapted to miniaturization and the probe can be in the form of an elongated tube of sufficient length and diameter to permit the tube to be slidably inserted through the channel of a nasogastric tube, an endoscope, or the like. Also, if desired, a thermistor temperature sensor can be incorporated in the probe to give a reading of core temperature at the pCO.sub.2 probe site, and allow the pCO.sub.2 reading to be corrected for temperature.
Further according to the invention, the pCO.sub.2 probe comprises a flexible and noncollapsible hollow tube which has an open end that defines inner and outer sidewalls. A containment structure for defining a closed reference chamber is coaxially disposed within the hollow tube. The containment structure comprises a selectively permeable glass membrane portion which is adjacent to the open end of the hollow tube, and a carrier portion extending axially within the hollow tube. A supporting and sealing means is disposed in the hollow tube, in sealing contact with the inner sidewall to support the carrier portion of the containment structure. The first electrode has a portion disposed in the reference chamber and a portion which extends through and is supported by the carrier means. The reference chamber is substantially filled with a first electrolyte of known H.sup.+ ion concentration.
A second electrode has a portion disposed concentrically around the glass membrane portion. The second electrode further has a portion extending through the supporting and sealing means. An end cap, comprising a second membrane selectively permeable to CO.sub.2, is carried by the outer sidewall of the tube, in sealing contact therewith. The end cap thereby defines a fluid tight electrolyte chamber within the tube. The fluid tight chamber so defined thereby also contains the glass membrane portion.
The end cap has a radially extending planar or curved end face with a longitudinally extending cylindrical sidewall. The cylindrical sidewall is orthogonal to the end face. The end cap is elastically held in place by tensile forces of the end face which urge the cylindrical sidewall into contact with the outer sidewall of the hollow tube. The electrolyte chamber is substantially filled with a second electrolyte capable of changing pH in response to changes in CO.sub.2 concentration.
For a more complete understanding of the invention and its objects and advantages, reference may be had to the following specification and to the accompanying drawings.